This invention relates to radars and more particularly to a transmitter, having a ripple and droop reduction circuit, for moving target indicator radars and pulse Doppler radars.
In a grid pulsed, depressed collector traveling-wave-tube (TWT) microwave amplifier or Klystron amplifier, the beam velocity is a function of the voltage from cathode to ground or body (beam voltage), and any change in voltage results in a corresponding RF phase shift through the tube. Thus, any power supply ripple appearing across the power supply capacitor will cause phase modulation of the amplified pulses. In many critical radar applications, excessive phase modulation of the output is highly undesirable.
In addition to the pulse-to-pulse phase changes caused by power supply ripple, there are phase and amplitude changes caused by the discharge of the power supply capacitor during the pulse.
In moving target indicator radars, and pulse Doppler radars the output pulses of the TWT amplifier must be substantially identical, in order that any variations in the return pulses are the result of the environment outside the radar.
In the past, the undesired ripple and droop in the beam supply was reduced by increasing substantially the size of the power supply capacitor. The problems attending the use of such a capacitor were the increased size and cost of the capacitor and the large amount of energy stored therein. As a rule of thumb, if the stored energy exceeds 50 joules, damage to the TWT results from tube arcing, unless a fast acting, expensive "electronic crowbar" circuit is provided. Also, in the past, a series regulation circuit was used to reduce the ripple and droop in the beam supply. A major problem with the series regulation circuit lies in obtaining good stability in the high gain, wide band amplifier and its associated sensing circuitry. The high voltages and high noise levels present in transmitter enclosures magnify this problem.
Accordingly, it is an object of the present invention to improve radars of the type using TWT of Klystron tube amplifiers.
Another object of the invention is to provide radars using TWTs or Klystron tube amplifiers with ripple and droop reduction circuits which are small, light and inexpensive to manufacture.
Still another object of the invention is to provide a ripple and droop reduction circuit having a gain of about one at the mid-range frequencies and less than one at other frequencies to negate any stabilization or oscillation problems.
Yet another object of the invention is to provide a ripple and droop reduction circuit in which the dc reference or supply voltages are not critical to its operation.
A further object of the invention is to provide a ripple and droop reduction circuit having inherent low gain and stability for operation remote from the high voltage compartment.
Briefly stated the invention comprises a ripple and droop reduction circuit for generating a ripple voltage substantially equal to but opposite in sign to the ripple voltage of the power supply for substantial cancellation of the ripple voltage effects on the stream of electrons in the TWT whereby the output pulses of the TWT have substantially the same characteristics.